Set up Windows build in our CI

[lgritz]

Our CI doesn't actually build and test on Windows. This causes no end of despair.

How hard can it be? You don't really need to know anything about OSL internals or even C++ programming. It's mainly about wrangling builds and knowing how such things are done on Windows. Maybe it's even a good Dev Days project?

OpenImageIO's build and CI system is basically the same as OSL's, and it has Windows builds in its CI, so that should provide much of the template (and existence proof) for how this can be done. Probably the hardest part is that unlike OIIO, OSL will need LLVM installed as a dependency.

[awesomebil]

Hello I would like to look into tackling this issue, I am a new developer and would just like to request some clarification. If I understand correctly the current build and install does not work on Windows and the goal is to get it working on Windows? Or am I missing something since it seems like according to the documentation building and testing on Windows should be possible as it is listed as supported although I have not actually tried to build the project on a Windows machine.

[lgritz]

We know that the software can build and run on Windows. But our project's GHA-based CI does not build and test on Windows. That is what we need to fix.